Abstract

Union Oil Company of Thailand, a subsidiary of Unocal Corporation, has developed major natural gas and condensate reserves in the Gulf of Thailand, approximately 130 miles south of Bangkok. Union, as operator on behalf of its co-ventures Southeast Asia Petroleum Exploration Company and Mitsui Oil Exploration Company Limited, has developed four offshore fields which are currently capable of producing 450 million cubic feet of gas and 20,000 producing 450 million cubic feet of gas and 20,000 barrels of con den sate per day. Natural gas from these fields supply 30 percent of Thailand's energy requirements. The Gulf of Thailand provides a challenging environment for primary cementing and cement evaluation. Most wells are directionally drilled, deviated up to 60, with an average total depth of 10,500', and have a bottom hole temperature of 320F or greater. Oil base mud is used routinely for drilling, and requires consideration when planning the cementing operation. Evaluation of the cementing operation is critical, as zones to be perforated are always in close proximity to water, either as gas water contacts or permeable water sands. In order to obtain a good evaluation of cement quality and distribution, the Cement Evaluation Tool (CET) has been used on over 30 wells to complement and improve the information obtained from the conventional Cement Bond Log (CBL). This paper shows examples of both CBL and CET on several wells in the Gulf of Thailand. Recently, the CET and CBL have been recorded simultaneously on one trip in the well, enabling a special presentation which displays the data on one film. These practical examples show how gas migration and channelling can be identified, in addition to the effects of casing centralization. The accuracy and resolution of the CET acoustic caliper for use in casing inspection is shown. Examples show the importance of the total circumferential cement distribution data provided by the CET in determination of zone isolation. The lack of microannulus effect on the CET is presented, along with recommendations on how to obtain good quality log data.

Introduction

Development wells in the Gulf of Thailand are directionally drilled from a 12-slot wellhead platform with a tender assisted drilling rig. A typical platform with a tender assisted drilling rig. A typical well is drilled to 8,500' total vertical depth (TVD) or 10,500' measured depth (MD) at an average angle of about 43 Several of the wells on a platform will exceed 50 and many have drop offs to near vertical, creating an "S" -shaped profile. After surface casing is set, a 12 "hole is directionally drilled in water base mud to 5000' MD. 9 5/8" casing is then run and cemented. The 8 "hole section is drilled to total depth utilizing non-toxic oil base mud and polycrystalline diamond compact bits. After logging, a full 7" casing string is run and cemented. A typical well schematic is shown in Figure 1. The productive gas sands are mainly spaced throughout the 8 "hole section, and are always close to producible water. A very severe cementing environment exists because of the "S" -shaped trajectory of the well, non-toxic oil base mud and high bottom hole temperatures. Evaluation of the cementing success in any production casing cement operation is important. production casing cement operation is important. Without sufficient isolation, sands can prematurely water out. The CET is another tool that gives more clues to both the quality of the cementation effort and the possible reasons for failure. The CET cannot make a bad cement job better; however, it may indicate whether or not sufficient isolation is obtained. Its insensitivity to microannulus and its 360 view around the casing give a clear indication of where the cement is located. In the Gulf of Thailand it has helped evaluate the centralizer program and other cementing variables to obtain good cement bonding. Gas migration through the setting cement has been eliminated as a major cause for poor bonding. One isolated case of gas migration is presented in the examples to show its effect on the CET.

CEMENT EVALUATION

After drilling, the wells are completed in a continous operation. The cement evaluation is usually finished one week or more after cement is in place. When possible, the cement evaluation is done without using rig time. By rigging up between the rig floor and the BOP deck, the logs are run while the rig is drilling on another well. Interference from the noise generated by the drilling operation has not affected the log quality. The logs are then sent to the office where they can be examined in detail and decision to squeeze can be made. Conventional CBL measurements of cement bond are circumferential averages of cement quality (Reference 1). The CET is a new approach to cement evaluation (Reference 2). It is a high frequency, ultrasonic device with 8 focused transducers evaluating the compressive strength of an azimuth of casing. Vertical resolution is improved due to the use of small ultrasonic transducers. The transducers act as transmitters and receivers, each emitting a short pulse of acoustic energy and then receiving the echo from the casing. In addition, the time between firing and reception of the first echo, associated with a measurement of the fluid transit time, enables the distance of each transducer from the casing to be determined. This yields four very accurate casing internal diameter measurements which are displayed for casing inspection. Recently it has been possible to run both CET and CBL simultaneously on one trip in the well. From the data of both measurements, an improved evaluation of cement strength and distribution is obtained. This added information has reduced the need for some squeeze operations, which has resulted in a savings of rig time and well cost. In order to obtain the best quality data the following are recommended:

Casing Fluid

Fluid must be present in the casing, preferably a clear brine. At the present time, CET data quality is reduced in 7" casing when the weight of a water based completion mud is greater than 12 lbs/gal. Oil base mud also reduces the CET signal level and is a further limitation. Several developments are expected to remove these limitations and improve the data quality in oil base and heavier muds.

Inside of Casing

In order for the CET to obtain maximum signal, the inside of the casing should be free of cement or debris. The current practice is to flush out the cement pumps and lines (clearing them of cement) before the cement is displaced (usually with seawater).

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